This invention relates to test and measurement instruments and, more particularly, to methods and apparatus for sampling and replicating the wave forms of electrical signals. Specifically, the invention is directed to a sampling signal analyzer for measuring, i.e., sampling and reconstructing for display, the non-transient or slowly-changing components and composite wave shape of an input signal based on ascertaining the frequency of the input signal and internally synthesizing an appropriate sampler drive signal, thereby obviating the need for a traditional high-speed trigger circuit responsive directly to the level of the input signal.
Heretofore, power meters have been used to measure the power of an input signal. Counters have been used to measure the fundamental frequency of an input signal. Spectrum analyzers have been used to measure the fundamental frequency and magnitude of an input signal and any harmonics that are present. However, none of these instruments has the capability of displaying the time domain (voltage versus time) wave shape of the input signal being measured.
In connection with measurement and display of non-transient or slowly-changing components and composite wave shape of an input signal, such as signals above 1 GHz, there is a need to record and examine fast rise-time (or fast fall-time) characteristics. One technique employed in the past is direct measurement of these input signals. Direct measurement requires an input signal trigger. Unfortunately, triggering in response to the level of the input signal is limited by the sensitivity and frequency response of the trigger circuit. Analog oscilloscopes used in the past have well-known problems associated with triggering in response to the level of the input signal, most notably being the trigger level sensitivity, trigger bandwidth, and trigger jitter. In general, the technology of triggering has not kept pace with that of sampling.
With regard to sampling, various data sampling instruments are known. For example, the block diagram of a typical sequential sampling digitizing oscilloscope is shown in FIG. 1. The sequential sampling digitizing oscilloscope is an architecture used to achieve high bandwidth.
In order to acquire measurement data, an input signal having a period T shown in FIG. 1 is routed through two separate electrical paths, namely, a high-frequency (HF) trigger circuit and a sampler circuit. The trigger circuit provides the correct timing of the sampler drive pulse relative to the level of the input signal. Once a trigger event is detected, an incremental delay circuit can delay for a short time before triggering the actual sampler drive pulse. Initially, however, triggering is not typically delayed, and the sampler drive pulse is generated directly from the trigger event in order to initiate acquisition of the first sample. Accordingly, the sampler circuit is enabled for a brief period of time and feeds a sampled analog voltage to an analog-to-digital converter (ADC). The digitized voltage is then processed by a microprocessor circuit and displayed on a display screen.
To summarize, the following is the sequence of events which occurs when a single sampled data point is acquired by the sequential sampling digitizing oscilloscope shown in FIG. 1. The input signal must satisfy a predetermined trigger condition. If so, a trigger pulse is generated by the HF trigger circuit. The sampler drive circuit enables the sampler circuit. Then, the output of the sampler circuit is digitized by the ADC. This sequence requires a finite length of time, for example, 0.1 millisecond. Accordingly, such sequential sampling digitizing oscilloscopes are constrained by the absolute speed limitations of the circuitry.
Furthermore, in order to display a wave form, more than one sampled data point is required. Therefore, the foregoing sequence is repeated with the following modifications. After the initially sampled data point has been digitized, the incremental delay circuit provides a delay after the trigger condition is again met. FIG. 2 shows how this modification will effect the sampling of the next data point. As shown in FIG. 2, each time that the delay time is lengthened, a new data point on the input signal wave form will be sampled. In fact, the delay time must be lengthened, or the same point of a steady-state periodic wave form will be repetitively sampled, which would mean that the wave shape could not be reproduced.
In view of the preceding discussion, the data acquisition operation of a sequential sampling digitizing oscilloscope can be characterized as triggering at a specific input signal level and then sampling the input signal in order to acquire the initial sampled data point. In order to acquire the next sampled data point, the sequential sampling digitizing oscilloscope triggers at the same level, but delays for a longer time and then samples the input signal.
The HF trigger circuit is instrumental in performing data acquisition. As each sampled data point is obtained, the wave shape of the input signal is progressively reconstructed.
However, in the case of sequential sampling digitizing oscilloscopes, the internal clock frequency can be, for example, 100 MHz. Generally, it must be determined when an asynchronous trigger occurs to a precision of about two percent of the input signal period. For an input signal frequency of 20 GHz, the triggering accuracy must be 1.0 picosecond, which is one part in 10,000 of the exemplary internal clock period. This accuracy is difficult to achieve.
Also, several cycles of the input signal wave form may occur between each sampled data point during the data acquisition process. For example, suppose that the sequential sampling digitizing oscilloscope receives a 100 MHz sine wave input signal. In the given example, approximately 0.1 millisecond is required to sample and digitize each data point. This means that the time between the first and second sampled data points is 0.1 millisecond. Since the period of a 100 MHz sine wave is 0.1E-7 second, 1E4 cycles of the input signal will occur between each of the data points, as shown in FIG. 3. Any change which occurs in the wave shape of the input signal during 10,000 cycles may not be measured.
Therefore, sequential sampling digitizing oscilloscopes are clearly limited, since they can require circuitry required to operate at speeds comparable to or higher than the input signal to be measured. Since this is not always possible, because of limitations of the speed of available data acquisition and digitizing circuitry, an alternative is needed to the traditional triggered sampled data acquisition process.
Also in the past, an instrument manufactured by Hewlett-Packard Company, headquartered in Palo Alto, CA, under the model designation HP 54100 has employed random repetitive data sampling, whereby triggered samples of a repetitive wave form are acquired every 25 nanoseconds if a given trigger level is reached. A trigger interpolator circuit determines where each sampled data point has occurred with respect to the trigger, i.e., whether it occurred before the trigger or after the trigger, and by how much before or after the trigger it occurred. Based on where the sampled data point occurred with respect to the trigger, a dot representing the position of the sampled data point in a voltage versus time relationship is directed to an output device, such as a sequential sampling digitizing oscilloscope display screen or a printer, where the dot is stored and/or displayed as an element of a reconstructed or synthesized wave shape. Due to the inherent shortcomings with respect to the accuracy of triggering in response to the level of an input signal, however, there is a limitation on the accuracy of such a synthesized wave form at high frequencies.
Finally, digital signal processing can be used to derive the frequency of an input signal from sampled data points, and the wave shape can be replicated, as described in U.S. Pat. No. 4,928,251. In accordance with the disclosure in this patent, representations of signal edges of a repetitive input signal being measured are sampled, next sorted out based on frequency and sequence, and then superimposed along a common time base of one period in order to reconstruct the input signal. More specifically, a string of samples of a repetitive input signal with high-frequency components is acquired with relatively low time resolution to determine an approximate wave shape from the low resolution samples, then digital signal processing, preferably in the form of a fast Fourier transform, is applied to a reconstructed time record of the input signal to obtain an accurate fundamental frequency, and finally the sampled wave form is reconstructed by overlaying sampled components with reference to a common time or phase reference. Further processing, for example, bin interpolation based on a window function, as described in U.S. Pat. No. 4,686,457, can be employed to improve the estimate of the fundamental frequency. Using values representing hundreds of samples of the input signal, it is possible to determine the wave shape, as well as its frequency, to an accuracy in excess of known trigger-based sampling techniques. However, this requires a substantial amount of data acquisition and digital signal processing capability.